Electric wave amplifier



Jan 8, 1952 L. J. HEAToN-ARMSTRONG 2,581,410

ELEcTRxc WAVE AMPLIFIER Filed March 3, 1949 2 SHEETS-SHEET 1 ATTORNFV Jan. 8, 1952 Filed March 3, 1949 ha 2gb E@ Y v L. J. HEATONARMSTRONG 2,581,410 ELECTRIC WAVE AMPLIFIER 2 Sl-lEETS-SHEET 2 ATTORNEY Patented Jan. 8, 1952 ELECTRIC WAVE AMPLIFIER Louis John Heaton-Armstrong, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application March 3, 1949, Serial N0. 79,466 In Great Britain March 5, 1948 4 Claims. l

The present invention relates to electric wave amplifiers and is concerned with the reduction of distortion produced by grid current in amplifiers having stages operated under class B conditions.

The distortion arises from the fact that the production of grid current in a power stage of the amplifier places an additional load on the input stages which increases with the signal amplitude, so that the effective gain of the amplifier decreases with increasing signal amplitude.

According to the invention, the distortion produced by grid current in this manner is effectively removed by feeding back to an earlier stage of the amplifier in positive phase the signal envelope obtained from the grid of the power stage which grid is arranged to act as a rectier when it is driven into the region of grid current.

In this specification the term amplifier may include amplifying arrangements with one or more frequency changing stages such as are commonly found in radio transmitters. It is, however, not essential that there should be any such frequency changing stages.

The invention will be described with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic circuit diagram of an embodiment of the invention;

Fig. 2 shows a graph used to explain the invention; and

Fig. 3 shows another embodiment of the invention.

Fig. 1 shows part of a high frequency amplifier including an input stage represented by a valve I, a power output stage represented by a valve 2, which is intended to be operated under class B conditions, and any number of conventionally arranged intermediate stages represented by the block 3, of which it is not necessary to show the details. Signal modulated carrier waves will be supplied at terminals 1i and 5, of which terminal 4 is connected to the control grid of the valve l,

.blocking condenser I3. A by-pass condenser I4 connects terminal to ground.

The power valve 2 is providedv with a directly quencies by the condenser 22. The anode of the valve 2 is connected to a pair of terminals 23 for the output load through a double tuned transformer 24, the primary tuning condenser being 25 and the secondary tuning condenser being divided into two equal sections 26 and 2l, the com.- mon point of which is grounded. The primary winding of the transformer 2d is connected to ground through a condenser 2B, and through resistance 29 shunted by an inductance coil 3Q to a terminal 3| for the high tension source (not shown) for the power valve 2, which may have lto supply a much higher potential than the r source for the valve nected to ground through a by-pass condenser The terminal 3| is con- 32. The elements 29 to 32 form a low-pass filter for preventing the carrier frequencies from being fed into the power source. The inductance 3i! is preferably wound over the resistance-29, thereby avoiding resonances in the inductance.

The full line A in Fig. 2 represents the usual relation between the input voltage at terminals 4 and 5 (abcissae) and the output voltage at terminals 23 (ordinates). This line is straight up to the point at which grid current commences, as indicated by the curve B, and then bends over as a result of the additional and increasing load which the grid current effectively produces on the output of the stages 3. Part of this effect may also be due to the fact that the anode voltage swing of the Valve2 is a large fraction of the applied high tension voltage. According to the invention, this effect is substantially eliminated by. connecting the upper end of resistance 2| to the junction point of the resistances 6 and I over conductor 33. The rectifying action of the grid is employed to produce with the aid of the elements 2| and 22 the rectified envelope of the modulated arrier wave, so that when the grid current increases, an increasing negative voltage will be applied to the cathode of the. valve The valve l should be of the kind such that an increase of the control grid potential with respect to the cathode increases the output power. Thus 'should have a directly heated cathode.

it will be seen thatl as the grid current increases, a compensating increase in the output power of the valve I can be produced, and the values of the resistances 6, 'I and 2I can be chosen so that the effective overall gain of the amplier remains substantially constant.

The condenser 20 should be large, so that the impedance of the source I9 will be reduced to a negligible value at the lowest modulating signal frequency. The resistance 2I should vpreferably be small (for example, of the order of 20 ohms) and the condenser 22 should be offsuch capacity that it presents a negligible impedance fat the carrier frequency, but has an impedance large compared with the resistance 2| at the highest modulating signal frequency.

In these circumstances, the control grid of the valve 2 operates as a normal rectifier. and a negative signal envelope voltage will be generated across the resistance 2l. This envelope voltage is applied across resistance 'I to thefcathode of the valve I, as already explained. The resistance 'i may be, for example, about 100 ohms,

While the resistance 6, vwhich provides the majority of thebias for the valve I, may be about 1000 ohms. The by-pass condenser '8 should have `a negligible impedance, at the carrier frequency, but a relatively high impedance at the highest signal frequency.

The valve 2 lhas been shown operated with a grounded grid, but this is not essential, since the same principles are applicable if the class B valve is operated in the normal way, with the cathode grounded. It is simply necessary to arrange the circuit so that the grid operates as a rectifier to produce the signal envelope, 'and to feed back the signal envelope in positive phase to an earlier stage of the amplier. The only requirement for the valve 2 is that 'the grid should not produce appreciable secondary emission, so that the grid current always increases with increase of input Voltage.

it is :also clearly not essential that the -valve 2 If an indirectly heated cathode is used, lthen condensers I and I1 are vnot necessary `and -the 'cathode will be connected directly tothe stages '53 'and-to the 'coil IB.

Fig$3 shows [another embodiment of theinvenvtion in which the amplier includes 'a .frequency changing stage, :and in which negative envelope feedback is also employed, according toth'eprinciples explained in the specification of my prior "application bearing Serial No. 753,149 Vviiled June 7, 1947,'now abandoned. Some Vof the yelements in Fig. 3 are the same'as corresponding elements in Fig. l, and have been given the same designation numbers'and 'do not need to be explained` again. n

In Fig. 3,-two`valve stages V34 Vand-35 areinterposed between'the input valve =I and the amplifying stages v3. VvThe valt/e331 isf'a phase changing 4valve, the anode of K which' is connected to terminal IIJ through Aa phasechanging network fconfsisting ofan inductance `?3B"in :serieswitharef lli lblocking condenser 50.

The conductor 33 is in this case connected to the movable contact of a two-way switch 5I by which it may be connected as desired either to 4the junction point of the resistances 3S) and i0 associated with the phase changing valve 34%, or tothe junction point of the resistances E and l'associated with rthe input valve I.

The anode of the valve 2 is connected to ground through two condensers 52 and 53 connected in series, while the junction point of condensers I5 and I1 is connected to ground through two further condensers 515, 55 also connected in series. The junction points of condensers 52,53 and 54, E35 are connected respectively to the fixed contacts of a second two-way switch 55. A conductor vE? connected to the movable contact of this switch leads to a second mixer or frequency vchanging valve 58, the anode oi which is connected directly to the anode of the input valve i. This forms the path over which negative envelope feedback is developed according to the principles explained in said prior application.

The conductor E? is connected to an intermediate point on the resistance 59 connected in series'between the cathode of the valve 53 and ground. The control grid is connected to ground through a leak resistance Si). An oscillator '6l supplies a heterodyne carrier wave to the control `grid of the valve'58 and also through a 'blocking condenser 62 to the upper end of the resistance d1 associated with the frequency changing valve `cuit i8 should preferably be broadly tuned to 29.2 megacycles thus selecting the upper sideband. The resistance i9 may be adjusted 'to secure suiiicient breadth of tuning to avoid serious phase'changes/at vthe vedges of 'the band asexplained in the specication already referred to.

The amplified band centred in 29.2 megacycles is tapped ofi either at the input or-a't the voutput of the'powervalve 2, according to the position of jthe switched andis fed back over conductor ffl'to the mixing valve 53 which recovers the original band centred on 3.1 megacycles 'and applies it tothe control grid of the 'valve Sdtogether with Atheoriginalinput band 'from the'valve l, but-.in opposite phase. The lphase changing network 36, 231, -38 should be 'adjustedto compensate for slight variations in phase in the amplier stages, so that the signal envelope is -fed back to thefvalve with a phase changeof exactly it` will thusl be seen that in addition to the positive envelope feedback supplied over conductor''B, 'according to the'present invention and as describe-:l with reference'to Fig. 1,`there is supplied negative"envelope feedback over conductor 5.' according to the principles described insaid vprior application.

the positive feedback path and therefore will reduce any distortion remaining after removing the effect of grid current in the power stage 2, according to the present invention.

It will be understood that although the valves have been shown as triodes for simplicity, tetrodes or pentodes may be used if desired, with the additional grids polarised in conventional ways. Furthermore, it is not essential that there should be any amplifying stages at all represented by the block 3, and if necessary for phase reversing reasons this block may be replaced by a transformer coupling.

While the principles of the invention have been described above in connection with specific ernbodiments, and particular modifications thereof, it is to be clearly understood that this descrip tion is made only by way of example and not as a limitation on the scope of the invention.

What is claimed is:

l. An electric wave amplifier comprising an input stage, a power stage including a tube having a grid electrode, said power stage adapted to be excited by said input stage, said grid electrode adapted to draw current when excited to a predetermined level,` an alternating current bypass condenser and a resistance connected in parallel and serially connecting said grid electrode to ground, a connection between said grid electrode and said input stage for feeding back a voltage in positive phase to compensate for the additional load on said input stage caused by the drawing of current of said grid electrode.

2. An electric wave amplifier as claimed in claim 1, further comprising a negative feedback circuit coupling said output stage and said input stage whereby distortion caused by the increased output of said input stage due to the feeding back of said positive voltage is compensated for. v

3. An electric wave amplifier as claimed in claim l, wherein said condenser and said resistance present a substantially zero impedance at the frequency of the wave to be amplied.

fi. An amplier according to claim 1 in which said input stage comprises a tube having a cathode and a resistance connected between said cathode and ground, said feedback connection comprising a connection between said resistances.

LQUIS JOHN HEATON-ARMSTRONG.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

